[adj] of or relating to the diaphragm; "phrenic nerve"
PHRENIC is a 7 letter word that starts with P.
Webster's 1913 Dictionary
Definition:
\Phren"ic\, a.[Gr. ?, ?, the midriff, or diaphragm, the
heart, the mind: cf. F. phr['e]nique.] (Anat.)
Of or pertaining to the diaphragm; diaphragmatic; as, the
phrenic nerve.
WordNet Dictionary